Electric power networks are prone to grid oscillations. For example oscillations may be seen in grid frequency, grid power, grid voltage angle and/or the speed of generators and other equipment used as part of a grid.
The disclosure herein refers principally to oscillations in grid frequency and oscillations in grid power. However, it should be noted that oscillations of other grid metrics are intentionally encompassed within the scope of the disclosure.
Oscillations in grid frequency can be prejudicial to the proper operation of an electrical grid. Oscillations in grid frequency may, for example, cause interconnector tripping, relay tripping, loss of load or generating plant stress. Such oscillations often involve (and affect) many generating plants. For example, frequency oscillations may be caused by rotor speed oscillations in generators. Also, such oscillations in grid frequency may span boundaries between network areas under the control of different operating companies or perhaps even national boundaries.
For the avoidance of doubt, it is noted that as used herein the term “oscillation in grid frequency” when relating to electrical power networks encompasses variations in a nominal frequency of the network. For example, an electrical power network may provide power at a frequency of 60 Hz. However, the frequency may vary between say 59 Hz and 61 Hz with a period of say 20 seconds, although this is an extreme example of oscillations in grid frequency. Therefore, the term “oscillation in grid frequency” refers to the periodic deviations over time of the frequency in an electrical power network, e.g. the periodic swings of the nominal 60 Hz grid frequency between 59 Hz and 61 Hz.
The variations in grid frequency and/or power also have an oscillation frequency associated with them.
The term “grid frequency” encompasses the frequency of power provided by the network, e.g. the nominal 50 Hz or 60 Hz frequency of the grid and any deviation from the nominal value.
Under certain network conditions, the oscillations in grid frequency may be poorly damped or even unstable. FIG. 1 illustrates an unstable oscillation in grid frequency that led to system separation and load-shed relay tripping. More specifically, FIG. 1 shows a graph of grid frequency over time, in which a nominal 60 Hz system oscillates at low frequency with a period of about 20 seconds, with swings from 59 Hz to 61 Hz.
The present inventors have appreciated that it is at present difficult to determine if a particular grid subsystem, such as a generating plant, control area or some defined region of the grid is contributing positively or negatively to the stability of a oscillation in grid frequency, or merely responding to the oscillation in grid frequency.
It is therefore an object for the present invention to provide an electrical grid comprising apparatus that is operable to determine if a grid subsystem is contributing positively or negatively to the stability of oscillation in grid frequency.
It is a further object for the present invention to provide a method of determining if a grid subsystem in an electrical grid is contributing positively or negatively to the stability of oscillations in grid frequency.